760 The American Naturalist. [September, 
quinquetubercular, with trigon and heel. The genus may be referred 
to a new family, the Thleodontide, with the following definition. 
Mandible without angle, but with inflected inferior border, and a 
coronoid process. Molars $-tubercular; premolais simple. Canines 
well developed ; incisors reduced. 
The discovery of this genus enables me to suggest a further reduc- 
tion on the number of genera named (but not described) by Prof. 
Marsh. Itis now indicated that the forms with only the simple and 
robust premolars which have been described by Marsh under the name 
of Stagodon, belong to the animals of which molars only are described 
under the names of Didelphops, Didelphodon, ete., and should be re- 
ferred to identical genera and species. What the simple-rooted tooth 
which served as the type of Marsh’s Stagodon really is, remains to be 
ascertained, but some of the premolars of Mammalia described by 
him under that name resemble those of Thleodon, although much 
inferior in size and less robust than are those of T. padanicus. The 
largest species described by Marsh, under the name of Stagodon 
validus' is not very different in size from the T. padanicus, but the 
number of premolars is probably smaller, or if equal, the anterior 
ones are longitudinal and not transverse. The description of Marsh is 
valuable as indicating the character of the incisors, a point not 
elucidated by my specimen. Marsh refers these forms to a family 
Stagodontide* which he does not define; moreover the generic charac- 
ter of the real Stagodon remains undescribed. 
The widely transverse condyle of Thlæodon shows that the move- 
ment of the lower jaw in mastication was vertical or orthal, as in the 
opossums, and not propalinal asin the Multituberculata, or loose as 
in the modern Monotremata. The true position of the family must, 
however, remain doubtful until other portions of the skeleton are 
discovered. The genus Thleodon may be simply a form of Didel- 
phyidz with simple robust premolars. 
Char. specif—The surface of attrition of the superior premolars is 
oblique to the vertical axis of the crowns, the latter spreading out- 
ward and downward in relation to the maxillary bone. The crown 
of the first premolar is very much larger than that of the second, and 
is subquadrilobate. This form results from the presence of three 
grooves which rise from the interradical spaces, but which do not 
attain the summit of the crown. The latter is obtusely rounded, with 
the anteroexternal diameter in excess of the anterointernal diameter. 
1Amer, Journ, Sci. Arts, 1889, August, p. 178. PI. vii, figs 22-5. 
2Op. cit. 1892, March 256. PI. viii, fig. 7. 
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